Magnetic resonance imaging (MRI) is commonly employed for medical imaging. In addition, combined use of MRI with radiation (X-Ray) therapy and with radiation imaging have both been used in a number of prior situations. Such systems provide significant advantages compared to single modality systems for obtaining information of the patient. However in such prior dual imaging systems, the patient may need to be moved or transferred from one system to another system. Such transfers can be difficult and time-consuming, and they can compromise results by complicating image registration.
MRI in combination with a radiotherapy accelerator has been used. Thus the X-Rays must be transmitted through the RF coil of the MR imaging system which remains in place during the radiotherapy. Although the RF coils of the MRI system are in the radiation path, they do not cause enough absorption to significantly degrade therapy or require a higher dose for imaging. For example the RF coils may have an equivalent AI thickness of about 2.3 mm, which is sufficiently low for the therapy to be carried out without interference from the coil and low enough not to affect the optimal dose of X-Ray radiation.
The combination of MRI with radiation imaging or more specifically X-Ray imaging causes unique problems. Specifically, if a conventional surface MRI Receive Coil is placed in the imaging path of a X-Ray system, the coil will be visible in the image and will cause artifacts or edges or components of the receive coil may cover important features in the X-Ray image which are required to be seen by the surgeon. For this reason, when MRI is performed in combination with radiation imaging, all coils of the MRI system, including the RF coils, are typically disposed out of or removed from the radiation path so as to be outside the field of view.
For example, U.S. Pat. No. 6,925,319 (McKinnon) issued Aug. 2, 2005 considers a split magnet MRI system having all MRI coils disposed out of the radiation path of an X-Ray system.
Unfortunately, MRI performance can be undesirably degraded by a requirement to place the MRI RF coils outside the field of view of a radiation imaging system. For example, surface RF coils are often placed directly on a subject being imaged for maximum MRI image quality. Such a surface coil is in the field of view of any radiation imaging system that is directed to the same part of the subject as the MRI system. Thus conventional combined MRI and radiation imaging can require an undesirable choice among accepting reduced MRI image quality by placing the RF coils out of the radiation system field of view, accepting RF coil artifacts in the radiation images by placing the RF coils in the radiation system field of view, or by moving the MRI RF coils to one position for MRI imaging and to another position out of the field of view for radiation imaging.
U.S. Pat. No. 7,394,254 (Rieke) issued Jul. 1, 2008 discloses using aluminum for the RF coils to render them “transparent” to X-Ray. More particularly, the patent discloses an arrangement in which improved compatibility of MRI with radiation imaging is provided by MRI RF coils having transmissive coil sections. The transmissive coil sections are substantially transparent to the penetrating radiation employed by the radiation imaging system. Thus the transmissive coil sections can be disposed in a field of view of the radiation imaging system without introducing artifacts into the radiation images. Transparency to penetrating radiation can be achieved by substantially including only low atomic number (i.e., Z<29) elements in the transmissive coil sections. Preferably, the transmissive coil sections are fabricated substantially from aluminum.
However it is accepted that the use of aluminum for the traces of an RF coil leads to a degradation in the MR imaging relative to the use of copper. However copper cannot be used in a trace which is sufficiently thin to generate the “transmissive” coil sections of Rieke.
Related disclosures are made in published PCT Applications WO 2009/146521 and 2009/146522 both filed May 25, 2009 and published Dec. 10, 2009 by the present Applicants/Assignees, the disclosures of which are incorporated herein by reference or to which reference may be made for further detail.
In PCT Application WO 2011/000085 filed Jun. 29, 2010 and published 6 Jan. 2011 by the present Applicants/Assignees is disclosed an RF coil where the conductive traces of the RF coil are arranged of a conductive material which has a thickness such that the traces cause an attenuation of the penetrating electromagnetic radiation which is visible in the radiation image and the support board includes non-conductive material in the field of view which has a thickness selected such that an attenuation of the penetrating electromagnetic radiation at locations on the board spaced from the traces is substantially equal to the attenuation at locations on the board at the conductive traces. However this arrangement does not disclose how to provide a phased array coil of this type where the attenuation is constant which has individual loops which, as is previously known, must overlap for decoupling mutual inductance between the coil loops. The disclosures of this document are incorporated herein by reference or to which reference may be made for further detail.
In PCT Application WO 2011/022896 published 3 Mar. 2011 by the present assignees is disclosed an RF coil loop including arrangements for decoupling and pre-amplification. The disclosures of this document are incorporated herein by reference or to which reference may be made for further detail.